It is known in the prior art that when a fuel cell power plant is shut down, with no electrical load across the cell, the reactant gases remaining in the fuel cell typically cause high cell voltages which in turn cause oxidation and corrosion of catalysts and catalyst supports. This in turn causes degradation of cell performance.
One manner to mitigate this problem is to purge the cell with inert gases, or nearly completely inert gases. In U.S. patent application Ser. No. 09/872,957, filed Jun. 1, 2001, a shut down procedure is postulated which requires containment of a particular gas mixture of hydrogen and principally nitrogen with possibly other gases that are inert and not harmful to the fuel cell. The nitrogen may be obtained from air, the oxygen of which is consumed early in the process by reacting with the hydrogen within the cell.
To maintain any particular gas composition in the fuel cell during a shut down period, it is necessary to close off the fuel and/or oxidant reactant flow field inlets and outlets.
In some atmospheric fuel cell power plants, no valves are utilized on the air flow. In other systems, only a single valve is used in the air flow. In order to maintain a particular gas composition within the oxidant flow fields, it is therefore necessary to add one or two valves to a fuel cell power plant.
In the case of the fuel reactant gas flow fields, two valves may typically be provided so that there is no problem in isolating the fuel gas flow fields during shut down, provided the expense and complexity of two valves are acceptable.